Air pollution is harmful not only to health but to electrical equipment. The corrosive components of polluted air, such as SO.sub.x and NO.sub.x, can corrode the electronic circuits of computers, wiring and junctions, thereby leading to disorders or errors.
A known countermeasure is to fit an adsorbing element packed with activated carbon into the vent holes or inside of electrical equipment to prevent pollutants from entering or to adsorb pollutants that have entered.
However, a conventional adsorbing element packed with powdered or granular activated carbon as such tends to generate electrically conductive dust. If the dust enters electrical equipment, it can induce a short-circuit. In addition, powdered or granular activated carbon that is merely packed into housing must be used in a large quantity to sufficiently remove pollutants. Therefore such an adsorbing element becomes bulky.
To overcome the above noted disadvantages associated with an adsorbing element using granular or powdered activated carbon, special measures are necessary to prevent dust from entering electrical equipment, including for example, tight sealing around the adsorbent with a porous membrane filter. In order to prevent dusting or improving adsorptivity, it has been proposed to set powdered activated carbon with a resin or to impregnate powdered activated carbon with a chemical. However, use of a resin-set powdered or granular activated carbon in an air flow path disadvantageously results in a large pressure drop, and those impregnated with a chemical suffer from release of the chemical into the surrounding equipment over time.
On the other hand, ACF has recently been developed. This is expected to be a promising adsorbent because of its high rate of adsorption based on a large outer surface area and superiority to powdered or granular activated carbon in adsorptivity per unit weight. Fabric or paper-like articles made of activated carbon fiber are bulky due to a low packing density of, for example about 0.1 g/cm.sup.3. Pieces of activated carbon fiber in a fabric or paper form tends to break off and generate dust. Fabric or paper-like activated carbon fiber is not easily fabricated into a desired shape and tends to break apart when cut into pieces to thereby generate dust. As a result, it is difficult to make fabric or paper-like activated carbon fiber into a compact adsorbent for use as an adsorbing element for electrical equipment. The above noted properties also limit size reduction of an adsorbing element. In a manner similar to the aforesaid powdered or granular activated carbon, fabric or paper-like activated carbon fiber must be tightly sealed to prevent when used as an adsorbing element for electrical equipment.
On the other hand, powdered or granular activated carbon is difficult to mold. Known molded articles of powdered or granular activated carbon include a honeycomb structure and a structure comprising activated carbon that is supported on an urethane article. In recent years, an activated carbon sheet comprising activated carbon powder and a fibrillating fluorine resin that forms fibrils has been proposed in JP-P-3-122008, JP-A-3-228813, and JP-A-3-228814 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"). According to these techniques, PTFE is fibrillated to form a network structure (of fibrils), and activated carbon particles are taken up into the network structure so as to retain their activity without being coated with PTFE.
A conventional ACF sheet has a low packing density, as low as about 0.1 g/cm.sup., due to the bulkiness of an ACF sheet. Thus, ACF molded articles disadvantageously have low packing density and are bulky. Moreover, an ACF has low fibrous strength and is brittle. Thus, when shaped into a sheet, the fibers break into fine particles and tend to fall off as black dust.
Paper formed from ACF should desirably have a high ACF content for ensuring high adsorptivity. However, an increase in ACF content causes an increase of black dust and a reduction in paper strength. For this reason, the upper permissible ACF content has been limited to 70 to 80 % by weight.
As an ACF-containing material for adsorbents, a filter sheet comprising ACF and rigid micro-fibrillated organic synthetic fiber such as fiber obtained from a poly(p-phenyleneterephthalamide) also has been proposed (see JP-A-3-202108). This filter sheet, which is prepared by previously mixing ACF and micro-fibrillated fiber and then making the mixture into paper, is basically a kind of paper. As a result the filter sheet lacks general-purpose processability.
When activated carbon powder is molded into a sheet together with a fibrillating fluorine resin as mentioned above, the packing density of the activated carbon powder is greately increased, and the resulting sheet like wise has an increased packing density. It follows that use of the adsorption sheet thus obtained in an air flow path causes an increased pressure drop. In order to ameliorate this drawback, a special means for controlling the packing density in an activated carbon sheet has been desired.
For example, the activated carbon sheet using powdered activated carbon, disclosed in JP-A-3-228813, is produced by kneading powdered activated carbon with a fibrillating fluorine resin, easily water-soluble inorganic salt powder, and an alcohol kneading aid, molding the mixture, and then dissolving and removing the inorganic salt to thereby reduce the packing density. The subject publication describes that the molded article produced by adding easily water-soluble inorganic salt powder has a bulk density (the same as a packing density) of 0.35 g/ml and exerts less pressure drop.
In order to reduce the packing density of an activated carbon sheet containing powdered activated carbon, JP-A-228814 teaches a method of preparing the sheet without using rollers for molding the sheet. In this method, the sheet is prepared by a paper making process and the sheet is subjected to a hot-pressing treatment at a temperature of 250.degree. C. or higher and a pressure of 10 kg/cm.sup.2 or higher. The publication discloses that the bulk density (the same as a packing density) of the thus produced activated carbon sheet is approximately less than 0.5 g/cm.sup.3, while a sheet prepared by using rollers has a bulk density of 0.65 g/cm.sup.3 and exerts a greater pressure loss.
However, the above-mentioned steps for reducing packing density tend to induce contamination of activated carbon with the salt or the alcohol used to prepare the sheet, and the resulting molded articles have limited porosity.